Powdered metal process

ABSTRACT

MALLEABLE METAL POWDERS HAVING LOW GREEN STRENGTH CAN BE CONVERTED TO HIGH GREEN STRENGTH, IRREGULARLY SHAPED PARTICLES ACCORDING TO A PROCESS COMPRISING MECHANICALLY WORKING, AS BY BALL MILLING, THE LOW GREEN STRENGTH POWDERS TO FLAKE FORM, ANNEALING THE FLAKES ABOVE THE RECRYSTALLIZATION TEMPERATURE THEREOF IN A NON-OXIDIZING ATMOSPHERE TO AGGLOMERATE THE FLAKE PARTICLES INTO A SINTER CAKE SUSCEPTIBLE OF SUBSEQUENT BREAK-UP AND MECHANICALLY DISINTEGRATING THE SINTER CAKE INTO THE IRREGULARLY SHAPED PARTICLES. ACCORDING TO THIS PROCESS, BLENDS OF LOW GREEN STRENGTH SPHERICAL ELEMENTAL POWDERS MAY BE CONVERTED TO HIGH GREEN STRENGTH, HIGH TRANSVERSE STRENGTH, HIGH HARDNESS ALLOY PARTICLES.

3,832,156 POWDERED METAL PROCESS Stanmore V. Wilson, Princeton, and Paul E. Matthews, Trenton, N.J., assignors to United States Bronze Powders, Inc.

No Drawing. Filed Sept. 27, 1972, Ser. No. 292,734 Int. Cl. B22f 1/00 US. Cl. 750.5 B 20 Claims ABSTRACT OF THE DISCLOSURE Malleable metal powders having low green strength can be converted to high green strength, irregularly shaped particles according to a process comprising mechanically working, as by ball milling, the low green strength powders to flake form, annealing the flakes above the recrystallization temperature thereof in a non-oxidizing atmosphere to agglomerate the flake particles into a sinter cake susceptible of subsequent brea'k-up and mechanically disintegrating the sinter ca'ke into the irregularly shaped particles. According to this process, blends of low green strength spherical elemental powders may be converted to high green strength, high transverse strength, high hardness alloy particles.

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to elemental and alloy powders having high green strength and, more particularly, to a method of making such powders.

Description of the Prior Art In the manufacture of metallic articles by powder metallurgical techniques, metal powders are often cold consolidated in an area physically remote from the sintering furnace, and thereafter transported to the furance. During transport the compact may be handled by mechanical apparatus, urged along conveyors, bumped, struck and generally subjected to rough treatment. Too frequently the compact chips, crumbles or otherwise deforms during handling because of the low green strength of the powders.

Heretofore, powder metallurgically formed articles, whether elemental or alloy, made from elemental powders, blends thereof or from prealloyed powders prepared by particulation of a homogeneous molten stream of metal with a fluid such as air, water, or inert gas having a spherical to modular morphology exhibited poor green strength. Cold consolidated compacts formed from this class of spherical to modular powders are easily crumbled or otherwise damaged as hereinbefore indicated.

SUMMARY OF THE INVENTION the present invention provides a process for forming high green strength particles suitable for powder metallurgical applications, which particles, by virtue of the herein described process, possess irregular shapes and highly desirable strength and hardness characteristics. According to the process of the present invention, malleable spherical elemental powders, blends of spherical elemental powders v 3,832,156 Patented Aug. 27, 1974 at least one which is malleable, and malleable pre-alloyed powders may be converted to a particle form which exhibits high green strength on cold consolidation. The process comprises mechanically working the powders to flake form, such as in a ball mill, annealing the flakes above the recrystallization temperature thereof in a non-, oxidizing atmosphere to form a sinter cake, and mechanically disintegrating the cake into irregularly shaped particles having high green strength. These high green strength particles may then be compacted and sintered according to conventional techniques to form the desired article.-

DETAILED DESCRIPTION OF THE INVENTION Spherical elemental powders, blends thereof, and prealloyed powders exhibit low green strength upon cold pressure compaction. It is the object of this invention to convert these powders to a physical form exhibiting high green strength. Thus the starting materials of the present process include commercially available metal powders, powder blends and pre-alloyed powders. Both ferrous and non-ferrous metal powders are contemplated as starting materials for the present process, the sole limitation being that at least one of the metals must be malleable. Where pure elemental powders or pre-alloyed powders are to be processed according to the present invention, these powders should be malleable. However, if a blend of elemental powders is to be processed, then only one metal powder of the blend need be malleable. For example, a malleable metal could be flaked milled, for example, with a grindable but brittle metal or even with a metal oxide to produce a diffused alloy.

Particle size of the starting material powders is not a critical consideration in the present process..Inasmuch as the present process does not generally reduce particle size, because it is not a grinding process, the particle size distribution of the starting material powders should approximate the desired size distribution of the resulting particles for the ultimate intended powder metallurgical use. Many powder metallurgical compacting-sintering procedures utilize particles having a size distribution in the range from about to smaller than 325 mesh, US. standard sieve, and therefore this process is particularly effective with such particles. However, it will be appreciated that any size particle useful in subsequent compacting-sintering operations may be processed according to the present process.

The starting material powders are blended, if necessary, and then are flaked to produce thin metal flakes. Generally, flaking is accomplished in a standard ball mill but any known method of flaking, such as with stamping machines, will be suitable. It is particularly noteworthy that the ball milling herein contemplated is not a grinding, comminuting or particle size reducing operation; it is not the intention to reduce the overall dimensions of a feed powder, but rather, to form a single element or multielement flake from pure metal powder, metal alloy powder, or mixture of elemental powders.

Ball milling, which is the primarily contemplated method of flaking, may be accomplished in commercially available ball mills utilizing ball sizes and operating at ball mill speeds which are readily ascertainable to achieve flaking rather than grinding. Optimum ball size generally depends upon the size of the starting material powders and upon the particle thinness or quality of flakes desired. Representative ball sizes are in the range from A3 to /2" diameter. Mill speed is another variable that depends to a large extent upon the particular mill, the ball size, the extent of flaking desired and a number of other factors. Generally, however, it is preferred to operate the mill at the speed at which maximum cascading occurs.v Experience has shown that this condition exists within the mill at :her.p.m.'at which Centrifugal force causes the'balls to follow the outer periphery of the mill rather than to cascade.

satisfactorily milling may be carried out in a circulating air atmosphere when non-pyrophoric metal powders or mixtures thereof are being milled. However, when milling pyrophoric powders, such as Cu Ni, the mill should be purged with a non-oxidizing atmosphere such as carbon dioxide or an inert gas. In addition, mill temperature is generally controlled, such as by external cooling of the mill with water.

Of particular importance, if flaking is to be accomplished, is the use of a ball milling lubricant to form protective boundary layers over the flake surfaces. Ball milling lubricants are well known and are commercially available. Exemplary of suitable lubricants are the fatty alcohols, fatty acids and the metallic stearates. A preferred and widely used lubricant is stearic acid. The amount of lubricant used depends upon the quantity of metal powder charged to the mill and the degree of flaking desired. Thus, as the extent of flaking increases, the surface area of the flakes increases and the amount of lubricant needed likewise increases. The optimum amount of lubricant for any mill, ball size and metal powder is best determined in trial runs, but representative lubricant quantities are set forth in the examples provided herein.

When processing a blend of metal powders to form alloy particles having high green strength, it is desirable to achieve a degree of mechanical bonding of the powders and cold welding of the flakes during ball milling. This is accomplished by a preliminary ball milling operation during which no lubricant is employed and therefore no appreciable flaking takes place. In the absence of the lubricant, no boundary layer is formed on the particles and mechanical interlocking thereof is enhanced. The preliminary mechanical interlocking step may be continued for about one half to three hours prior to flaking in the mill in the presence of a lubricant. Less than one half hour of non-lubricant milling has been found to produce too little interlocking to be meaningful, while non-lubricant milling for more than three hours causes grinding and particle size reduction to occur. Preferably, the preliminary non-lubricant milling of blended elemental powders is carried on for about one hour. As a result of the nonlubricant milling, the mechanically interlocked powders experience some small degree of alloying and cold welding during flaking.

The flakes resulting from milling of the powders are highly stressed and strained, and are brittle due to the cold working. In order to recrystallize, so soften, and to stress relieve the flakes, they are annealed following ball milling. Prior to annealing, it may be desirable to classify the flakes, as by screening, to insure that any oversize flakes which may be present are removed. Annealing may be accomplished in a standard annealing furnace at temperatures above the recrystallization temperature of the flakes. Inasmuch as annealing is a time-temperature process,-and since the properties of different metal flakes vary widely, it would be meaningless to set forth a temperature range or time range for the annealing. Suflice to say that the purpose of the annealing is to homogenize or solution treat the flakes, to stress relieve and soften them, under the most effective, yet economical, conditions possible. Tests have shown that annealing is preferably carried to the point where the flakes agglomerate to form a sinter cake which is readily disintegratable into irregular particles by mild mechanical means. The annealing furnace atmosphere should be non-oxidizing, and, inasmuch as the starting material powders doubtless include some oxides, the atmosphere is preferably reducing to eliminate the oxides. One common and suitable reducing atmosphere is a dissociated ammonia atmosphere, although it will be appreciated that numerous reducing atmospheres for annealing furnaces are Well known.

It is noteworthy that when the present process is employed to convert an elemental powder"blend to'atru'e alloy by diffusion, the bulk of the alloying takes place during the anneal. Thus the mechanically interlocked, cold welded, thin flakes exiting from the ball mill diffuse into each other during the anneal to produce a homo geneous alloy. Most important is that thediffusionoccu'rs at a rapid rate. If the starting material metalp'owders had merely been mixed together and then sintered, some diffusion would take place, but it would require long periods of time and complete diffusion could probably never be achieved.

Following annealing, the resulting sinter cake is subjected to mild mechanical disintegration in conventional pulverizing apparatus to break-up the cake into irregular particles. The pulverization is manifestly not a grinding operationbut rather a technique for imparting the desired physical irregularity to the flakes, i.e., a particle shaping operation. In fact, particle size distribution following disintegration has been found to be generally shifted slightly toward larger particles than were present in the starting material powders. This observation confirms that neither the flake milling step nor the pulverizing step involve grinding or particle size reduction. 1

The irregular particles which are the product of the present process are eminently well suited for use in conventional powder metallurgical techniques. Thus, they can be cold consolidated in a die to the shape of the ultimate article and, in such form, exhibit high green strength. Following such compacting the particles may be sintered, by well known techniques, to form a final, high-density, metallic article.

EXAMPLE I Mono-metallic spherical, ductile copper powder manufactured by particulation from a molten stream with air was converted to irregular particles according to the present process. Spherical copper, U.S. Bronze Powders, Inc. Grade #2 (percent Cu 99.2 min.) having a bulk density of 4.56 grams/ cc. was used as the starting material powder. The copper powder exhibited a green strength on cold consolidation to 7.7 grams/cc. of less than 10 p.s.i. and had a particle size distribution according to the following screen analysis:

Percent, by weight,

retained mesh 0.1 mesh 0.2 mesh 6.1 +200 mesh 11.0 +325 mesh 22.3 Pan 60.3

The starting material copper powder was ball milled in a continuous type mill using diameter balls, a ball mill speed of about 54 rpm. and a recirculating air atmosphere. About 0.36% by weight of stearic acid was added with the feed metal powder. The mill was externally cooled with water. The resulting flakes which had abulk density of 0.6 gram/cc, were annealed in a Drever furnace in a dissociated ammonia atmosphere for 15 minutes at 1650 B, after which the resulting cake was passed through a Weber Brothers pulverizing mill and the pulverized powder hand screened through an 80 mesh U.S. standard sieve. The resulting irregular particles had a bulk density of 2.08 grams/cc, a green strength at 7.77 grams/ cc. of 5,840 p.s.i. and a screen analysis as follows:

Percent, by weight,

EXAMPLE II Percent, by weight,

retained 80 mesh Trace +100 mesh Trace +140 mesh Trace +200 mesh 12.4 +325 mesh 26.2 Pan 61.4

The starting material bronze powder was ball milled in a batch type mill using 01 diameter balls and a mill speed about 44 r.p.m. The mill was purged with carbon dioxide and was not cooled externally. About 0.4% by weight stearic acid was added with the metal powder feed. Milling was continued for 6 hours after which the resulting flakes had a bulk density of 2.32 grams/cc. The flakes were annealed in a Drever furnace for 15 minutes at 1400 F. using a dissociated ammonia atmosphere. Following annealing the sinter cake was pulverized in a Weber Brothers pulverizing mill and hand screened through a 60 mest U.S. standard sieve. The resulting irregular particles had a bulk density of 1.76 grams/cc, a green strength at 7.47 grams/cc. of 1485-1510 psi. and a screen analysis as follows:

Percent, by weight,

retained 60 mesh 5.5 80 mesh 9.2 +100 mesh 11.3 +140 mesh 22.6 +200 mesh 24.9 +325 mesh 15.0 Pan 14.5

EXAMPLE III A bi-elemental powder blend of copper and nickel, in the proportions of 75% by weight copper to 25% by weight nickel, was converted to an alloy powder according to the present invention. Starting materials were U.S. Bronze Powders, Inc. Grade C-l32 copper powder and Alcan Metal Powders MD-lOl nickel powder. The bulk densities of the copper and nickel powders were 2.45 grams/cc. and 3.70 grams/cc. respectively. The powders had a particle distribution as shown .by the following screen analysis:

Percent, by weight retained- Copper Nickel powder powder +80 mesh Trace 0 +100 Trace 0.1 1.1 12. 9 5. 7 17. 0 29. 2 25. 0 64. 0 45. 0

0.5% by weight stearic acid was added to the mill based on the weight of metal powder in the mill. The mill was repurged with carbon dioxide and restarted. Following eight hours total milling time the bi-metal flake product exhibited a bulk density of 1.34 grams/cc. The flakes were annealed in a Drever furnace for 30 minutes at 1800 F. in a dissociated ammonia annealing atmosphere. Following annealing the flakes were precrushed using a mortar and pestle and pulverized in a Weber Brothers pulverizing mill, after which they were hand screened through a -U.S. standard sieve 60 mesh. The resulting particles had a bulk density of 2.39 grams/cc. and a particle distribution as evidenced by the following screen analysis:

Percent, by weight,

retained 60 mesh 0.4

+ 80 mesh 12.1 +100 mesh 12.9 +140 mesh 23.3

+200 mesh 24.2 +325 mesh 17.0 Pan 10.1

The irregular particle shaped alloy product was comparatively tested against the 75% copper/ nickel elemental powder blend starting material and against the powder blend starting material annealed for minutes at 1800 F. in a dissociated ammonia annealing atmosphere. By blending each of the starting material powder blend, the annealed powder blend and the particles prepared by the present process with by weight, lithium stearate and subjecting each blend to a compacting pressure of 30 tons/sq. in. to achieve a compacted density of 7.6 grams/cc. the green strength, transverse strength and Rockwell Hardness could be tested by standard test procedures. The transverse strength and Rockwell Hardness tests were conducted on Metal Powder Industries Federation standard 1.25 x 0.50 x 0.25 inch compacted transverse bars sintered for 30 minutes at 2000 F. in a dis- 4 sociated ammonia atmosphere.

Blend- Present 75% Cu, 25% Ni Blend annealed process Green strength (lbs/in!) 1,480 1,137 1,938 Transverse strength (lbs/in?) 26,100 28,700 58, 100 Rockwell hardness H 36 H 37 H 74 While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications can be made by those skilled in the art without actually departing from the scope of the invention. Accordingly, all suitable modifications and equivalents may be resorted to which fall within the scope of the invention as claimed.

What is claimed as new is as follows:

1. A process for forming irregular metal particles exhibiting high green strength on cold consolidation, comprising the steps of:

(a) mechanically working low green strength metal particles selected from the group consisting of malleable elemental powders, mixtures of elemental powders at least one of which is malleable, and malleable prealloyed powders in the presence of a lubricant to flake said particles;

(b) annealing said flake particles above the recrystallization temperature thereof in a non-oxidizing atmosphere for a time and at a temperature suflicient to agglomerate the flake particles into a sinter cake susceptible of subsequent break-up; and

(c) mechanically disintegrating said sinter cake intofor their ultimate intended use.

4. A process, as claimed in claim 1, wherein a major proportion of said low green strength particles pass a 200 mesh screen.

5. A process, as claimed in claim 2- wherein said metal particles are a blend of elemental powders at least one of which is malleable and including the additional step of ball milling said particles in the absence of a lubricant for a period from one half to three hours prior to ball milling in the presence of a lubricant.

6. A process, as claimed in claim 2, wherein said ball milling is accomplished in a non-oxidizing atmosphere.

7. A process, as claimed in claim 6, wherein said nonoxidizing atmosphere is an inert atmosphere.

8. A process, as claimed in claim 1, wherein said annealing is accomplished in a reducing atmosphere.

9. A process, as claimed in claim 1, including the additional step of classifying said mechanically worked flakes prior to annealing.

10. A process, as claimed in claim 1, wherein said lubricant is selected from the group consisting of fatty acids, fatty alcohols and metallic stearates.

11. A process, as claimed in claim 10, wherein said lubricant is stearic acid.

12. A process for forming irregular metal particles exhibiting high green strength on cold consolidation comprising the steps of:

(a) ball milling low green strength metal particles selected from the group consisting of malleable elemental powders, blends of elemental powders at least one of which is malleable, and malleable prealloyed powders, having a particle size distribution satisfactory for their ultimate intended use, in the presence of a lubricant to flake said particles;

(b) annealing said flake particles above the recrystallization temperature thereof in a non-oxidizing atmosphere for a time and at a temperature sufiicient to agglomerate the flake particles into a sinter cake susceptible of subsequent break-up; and

(c) mechanically disintegrating said sinter cake into particles having irregular shapes.

13. A process, as claimed in claim 12, wherein said metal particles are a blend of elemental powders at least one of which is malleable and including the additional step of ball milling said particles in the absence of a lubricant for a period from one half to three hours prior to ball milling in the presence of a lubricant.

14. In a process for forming a metal article by powder metallurgical techniques including the steps of compacting metal particles to the shape of the desired article and sintering the compacted particles, the improvement comprising forming irregular shaped, high green strength particles prior to compacting according to the following steps:

(a) ball milling the metal particles in the presence of a lubricant to fiake said particles;

(b) annealing said flake particles above the recrystallization temperature thereof in a non-oxidizing atmosphere for a time and at a temperature sufficient to agglomerate the flake particles into a sinter cake susceptible of subsequent break-up; and p (c) mechanically disintegrating said sinter cake into particles having irregular shapes.

15. A process, as claimed in claim 14, wherein said metal particles have a size distribution suitable for their ultimate intended use, and are selected from the group consisting of malleable elemental powders, blends of elemental powders at least one of which is malleable, and malleable pre-alloyed powders.

16. A process, as claimed in claim 14, wherein said metal particles are a blend of elemental powders at l ast one of which is malleable and including the additional step of ball milling said particles in the absence of allu bricant for a period from one half to three hours prior to ball milling in the presence of a lubricant. j p

17. A process, as claimed in claim 6, wherein said metal particles are a blend of copper andnickel powders.

18. A process, as claimed in claim 17, wherein a major proportion by weight of said powder blend is copper.

19. A process, as claimed in claim 18, wherein said powder blend consists essentially of by weight copper and 25% by weight nickel.

20. The alloy product produced by the process of claim References Cited UNITED STATES PATENTS 3,212,876 10/1965 Hulthn 148- -126 3,723,092 3/ 1973 Benjamin 750.5 R 2,689,398 9/1954 Gaut et al. 148126 2,860,044 11/ 1958 Brundin 75-0.5 BA 2,902,357 9/ 1959' Crooks et al. 750.5 BA

WAYLAND W. STALLARD, Primary Examiner U.S. Cl. X.R. 

